This invention relates generally to nuclear reactors, and more particularly to jet pump slip joint ovalization for boiling water nuclear reactors.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
In a BWR, hollow tubular jet pumps positioned within the shroud annulus, provide the required reactor core water flow. The upper portion of the jet pump, known as the inlet mixer, is laterally positioned and supported against two opposing rigid contacts within restrainer brackets by a gravity actuated wedge. The restrainer brackets support the inlet mixer by attaching to the adjacent jet pump riser pipe. The lower portion of the jet pump, known as the diffuser, is coupled to the inlet mixer by a slip joint. The slip joint between the jet pump inlet mixer and the jet pump diffuser collar has about 0.015 inch diametral operating clearance which accommodates the relative axial thermal expansion movement between the upper and lower parts of the jet pump and permits leakage flow from the driving pressure inside the pump.
Excessive leakage flow can cause oscillating motion in the slip joint, which is a source of detrimental vibration excitation in the jet pump assembly. The slip joint leakage rate can increase due to single loop operation, increased core flow, or jet pump crud deposition. The restrainer bracket laterally supports the inlet mixer through three point contact provided by two set screws and the inlet mixer wedge at an elevation above the slip joint. Set screw gaps can occur during plant operation. Sometimes, the inlet mixer appears to settle to a position away from the set screw, while in other cases, wear occurs between the mixer wedge and the restrainer pad. In both cases, three point contact is lost and the potential for vibration is significantly increased. Set screw gaps are affected by the difference in thermal and pressure displacements of the shroud, pressure vessel, and rotation of the shroud support plate. In addition to affecting set screw gaps, thermal and pressure displacements of the shroud and the pressure vessel can diminish alignment interaction loads in the jet pump assembly which are beneficial in restraining vibration, such as a lateral force in the slip joint. The resultant increased vibration levels and corresponding vibration loads on the piping and supports can cause jet pump component degradation from wear and fatigue.
High levels of flow induced vibration (FIV) is possible in some jet pump designs at some abnormal operational conditions having increased leakage rates. Therefore, it is desirable to provide a jet pump assembly that that has a lateral load in the slip joint area to maintain rigid contact between the inlet mixer and the diffuser collar to prevent oscillating motion and suppress FIV.
A method for applying a lateral support load to a jet pump slip joint in accordance with an exemplary embodiment of the present invention includes creating an oval deformation of the jet pump diffuser. The jet pump includes a jet pump inlet mixer and a jet pump diffuser joined together by a slip joint. A bottom end of the inlet mixer is inserted into a top end of the diffuser to form the slip joint. The wall of the inlet mixer having a smaller thickness than the wall of the diffuser.
The method includes positioning an ovalization device around the diffuser and actuating the ovalization device to apply a predetermined load to the slip joint which creates an oval deformation of the diffuser. The force applied by the ovalization device creates a plastic strain in the diffuser wall which permits the diffuser to maintain an oval shape. Because of the thinner wall thickness of the inlet mixer, the applied force produces an elastic strain in the wall of the inlet mixer, which then attempts to restore its original circular shape when the load applied by the ovalization device is released. The elastic deflection of the inlet mixer as the mixer moves to its original shape applies a lateral preload force to the diffuser at the area where the diffuser has a reduced diameter due to the oval deformation. This lateral preload force maintains a rigid contact between the inlet mixer and the diffuser collar to prevent oscillating motion and suppress FIV. Also, the deformation is controlled so that the elastic deformation induced preload force is sufficient to prevent vibratory motion in the slip joint but does not cause excessive friction in the slip joint so as not to interfere with assembly and disassembly of the slip join or the required sliding to accommodate operating thermal expansion displacements.